EN 300 966 V6.0.1 (1999-06) European Standard ( series)

Digital cellular telecommunications system (Phase 2+); Half rate speech; Half rate speech processing functions (GSM 06.02 version 6.0.1 Release 1997)

R GLOBAL SYSTEM FOR MOBILE COMMUNICATIONS GSM 06.02 version 6.0.1 Release 1997 2 EN 300 966 V6.0.1 (1999-06)

Reference DEN/SMG-110602Q6 (8y00300o.PDF)

Keywords Digital cellular telecommunications system, Global System for Mobile communications (GSM)

ETSI

Postal address F-06921 Sophia Antipolis Cedex - FRANCE

Office address 650 Route des Lucioles - Sophia Antipolis Valbonne - FRANCE Tel.: +33 4 92 94 42 00 Fax: +33 4 93 65 47 16 Siret N° 348 623 562 00017 - NAF 742 C Association à but non lucratif enregistrée à la Sous-Préfecture de Grasse (06) N° 7803/88

Internet secretariat@.fr Individual copies of this ETSI deliverable can be downloaded from http://www.etsi.org

Copyright Notification

No part may be reproduced except as authorized by written permission. The copyright and the foregoing restriction extend to reproduction in all media.

© European Telecommunications Standards Institute 1999. All rights reserved.

ETSI GSM 06.02 version 6.0.1 Release 1997 3 EN 300 966 V6.0.1 (1999-06)

Contents

Intellectual Property Rights...... 4 Foreword ...... 4 1 Scope...... 6 2 References...... 6 3 Definitions and abbreviations ...... 7 3.1 Definitions ...... 7 3.2 Abbreviations...... 7 4 General...... 8 5 Half rate speech transcoding...... 9 6 Half rate Discontinuous Transmission (DTX)...... 9 7 Half rate Voice Activity Detection (VAD)...... 10 8 Half rate comfort noise insertion ...... 10 9 Half rate lost speech frame substitution and muting...... 10 10 Half rate homing ...... 11

Annex A (informative): Change Request History ...... 12 History...... 13

ETSI GSM 06.02 version 6.0.1 Release 1997 4 EN 300 966 V6.0.1 (1999-06)

Intellectual Property Rights

IPRs essential or potentially essential to the present document may have been declared to ETSI. The information pertaining to these essential IPRs, if any, is publicly available for ETSI members and non-members, and can be found in SR 000 314: "Intellectual Property Rights (IPRs); Essential, or potentially Essential, IPRs notified to ETSI in respect of ETSI standards", which is available free of charge from the ETSI Secretariat. Latest updates are available on the ETSI Web server (http://www.etsi.org/ipr).

Pursuant to the ETSI IPR Policy, no investigation, including IPR searches, has been carried out by ETSI. No guarantee can be given as to the existence of other IPRs not referenced in SR 000 314 (or the updates on the ETSI Web server) which are, or may be, or may become, essential to the present document.

Foreword

This European Standard (Telecommunications series) has been produced by ETSI Technical Committee Special Mobile Group (SMG).

The present document gives a general overview of the half rate speech traffic channels speech processing functions for the digital cellular telecommunications system. The present document is part of ENs' series covering the half rate speech traffic channels as described below:

GSM 06.02 "Digital cellular telecommunications system (Phase 2+); Half rate speech; Half rate speech processing functions".

GSM 06.20 "Digital cellular telecommunications system (Phase 2+); Half rate speech; Half rate speech transcoding".

GSM 06.21 "Digital cellular telecommunications system (Phase 2+); Half rate speech; Substitution and muting of lost frames for half rate speech traffic channels".

GSM 06.22 "Digital cellular telecommunications system (Phase 2+); Half rate speech; Comfort noise aspects for half rate speech traffic channels".

GSM 06.41 "Digital cellular telecommunications system (Phase 2+); Half rate speech; Discontinuous Transmission (DTX) for half rate speech traffic channels".

GSM 06.42 "Digital cellular telecommunications system (Phase 2+); Half rate speech; Voice Activity Detector (VAD) for half rate speech traffic channels".

GSM 06.06 "Digital cellular telecommunications system (Phase 2+); Half rate speech; ANSI-C code for the GSM half rate speech codec".

GSM 06.07 "Digital cellular telecommunications system (Phase 2+); Half rate speech; Test sequences for the GSM half rate speech codec".

The contents of the present document is subject to continuing work within SMG and may change following formal SMG approval. Should SMG modify the contents of the present document it will be re-released with an identifying change of release date and an increase in version number as follows:

Version 6.x.y

where:

6 indicates Release 1997 of GSM Phase 2+

x the second digit is incremented for all changes of substance, i.e. technical enhancements, corrections, updates, etc.

y the third digit is incremented when editorial only changes have been incorporated in the specification.

ETSI GSM 06.02 version 6.0.1 Release 1997 5 EN 300 966 V6.0.1 (1999-06)

Proposed national transposition dates Date of adoption of this EN: 05 June 1999 Date of latest announcement of this EN (doa): 30 September 1999 Date of latest publication of new National Standard or endorsement of this EN (dop/e): 31 March 2000 Date of withdrawal of any conflicting National Standard (dow): 31 March 2000

ETSI GSM 06.02 version 6.0.1 Release 1997 6 EN 300 966 V6.0.1 (1999-06)

1 Scope

The present document is an introduction to GSM 06.20 [4], GSM 06.21 [5], GSM 06.22 [6], GSM 06.41 [7] and GSM 06.42 [8] ETSs dealing with the speech processing functions in the half-rate channel of the GSM system. A general overview of the speech processing functions is given, with reference to the ETSs where each function is specified in detail.

2 References

The following documents contain provisions which, through reference in this text, constitute provisions of the present document.

• References are either specific (identified by date of publication, edition number, version number, etc.) or non-specific.

• For a specific reference, subsequent revisions do not apply.

• For a non-specific reference, the latest version applies.

• A non-specific reference to an ETS shall also be taken to refer to later versions published as an EN with the same number.

[1] GSM 01.04: "Digital cellular system (Phase 2+); Abbreviations and acronyms".

[2] GSM 03.50: "Digital cellular telecommunication system (Phase 2+); Transmission planning aspects of the speech services in the GSM Public Land Mobile Network (PLMN) system".

[3] GSM 05.03: "Digital cellular telecommunications system (Phase 2+); Channel coding".

[4] GSM 06.20: "Digital cellular telecommunications system (Phase 2+); Half rate speech: Half rate speech transcoding".

[5] GSM 06.21: " Digital cellular telecommunications system (Phase 2+); Half rate speech; Substitution and muting of lost frame for half rate speech traffic channels".

[6] GSM 06.22: "Digital cellular telecommunications system (Phase 2+); Half rate speech; Comfort noise aspects for half rate speech traffic channels ".

[7] GSM 06.41: "Digital cellular telecommunications system (Phase 2+); Half rate speech; Discontinuous transmission (DTX) for half rate speech traffic channels".

[8] GSM 06.42: "Digital cellular telecommunications system (Phase 2+); Half rate speech; Voice Activity Detector (VAD) for half rate speech traffic channels ".

[9] ITU-T Recommendation G.711: "Pulse Code Modulation (PCM) of voice frequencies".

[10] GSM 06.07: "Digital cellular telecommunications system (Phase 2+); Half rate speech; Test sequences for the GSM half rate speech codec".

[11] GSM 06.06: "Digital cellular telecommunications system (Phase 2+); Half rate speech; ANSI-C code for the GSM half rate speech codec".

ETSI GSM 06.02 version 6.0.1 Release 1997 7 EN 300 966 V6.0.1 (1999-06)

3 Definitions and abbreviations

3.1 Definitions

Definition of terms used in the present document can be found in GSM 06.20 [4], GSM 06.21 [5], GSM 06.22 [6], GSM 06.41 [7] and GSM 06.42 [8].

3.2 Abbreviations

For the purposes of the present document, the following abbreviations apply.

BFI Bad Frame Indication BSS Base Station System DTX Discontinuous Transmission GSM Global System for Mobile communications MS Mobile Station PCM Pulse Code Modulated PLMN Public Land Mobile Network PSTN Public Switched Telephone Network RF Radio Frequency RX Receive RSS Radio SubSystem SACCH Slow Associated Control CHannel SID SIlence Descriptor SP flag SPeech flag TAF Time Alignment Flag TX Transmit VSELP Vector Sum Excited Linear Predictor UFI Unreliable Frame Indication

For abbreviations not given in this subclause, see GSM 01.04 [1].

ETSI GSM 06.02 version 6.0.1 Release 1997 8 EN 300 966 V6.0.1 (1999-06)

4 General

Figure 1 presents a reference configuration where the various speech processing functions are identified. In this figure, the relevant documents for each function are also indicated.

In figure 1, the audio parts including analogue to digital and digital to analogue conversion are included, to show the complete speech path between the audio input/output in the Mobile Station (MS) and the digital interface of the Public Switched Telephone Network (PSTN). The detailed specification of the audio parts are contained in GSM 03.50 [2]. These aspects are only considered to the extent that the performance of the audio parts affect the performance of the speech transcoder

TX DTX handler GSM 06.42 GSM 06.41 Voice Activity 3 SP Detector BSS side only GSM 06.02 VAD flag

8 bit / A - law 6 to GSM 06.20 1 13 bit uniform 6 DTX Speech control 2 encoder 4 and info. Speech operation bits fram e LPF A/D 7 GSM 06.22 MS side only GSM 03.50 Comfort noise 5 TRANSMIT SIDE TX functions SID frame RX DTX handler Info bits GSM 06.41 GSM 06.21 Speech 8 fram e BFI substitution BSS side only GSM 06.02 9 13 bit uniform Speech GSM 06.20 to 1 SID DTX frame 8 bit/A-law control speech 10 2 and 4 decoder operation TAF D/A LPF

11 SID GSM 06.22 frame MS side only GSM 03.50 UFI Comfort noise 5 12 RX functions RECEIVE SIDE

1 8 bit /A-law PCM (ITU-T Recommendation G.711) [9], 8 000 samples/s.

2 13 bit uniform PCM, 8 000 samples/s.

3 Voice Activity Detector (VAD) flag.

4 Encoded speech frame, 50 frames/s, 112 bits/frame.

5 SIlence Descriptor (SID) frame, 112 bits/frame.

6 SPeech (SP) flag, indicates whether information bits are speech or SID information.

7 Information bits delivered to the radio subsystem.

ETSI GSM 06.02 version 6.0.1 Release 1997 9 EN 300 966 V6.0.1 (1999-06)

8 Information bits received from the radio subsystem.

9 Bad Frame Indication (BFl) flag.

10 SIlence Descriptor (SID) flag.

11 Time Alignment Flag (TAF), marks the position of the SID frame within the Slow Associated Control CHannel (SACCH) multiframe.

12 Unreliable Frame Indication (UFI).

Figure 1: Overview of audio processing functions

5 Half rate speech transcoding

The half rate speech transcoding function is described in GSM 06.20 [4].

As shown in figure 1, the speech encoder takes its input as a 13 bit uniform Pulse Code Modulated (PCM) signal either from the audio part of the MS or on the network side, from the PSTN via an 8 bit/A-law to 13 bit uniform PCM conversion. The encoded speech at the output of the speech encoder is delivered to the channel coding function as defined in GSM 05.03 [3] to produce an encoded block consisting of 228 bits leading to a gross bit rate of 11,4 kbit/s.

In the RX direction, the inverse operations take place.

GSM 06.20 [4] describes the detailed mapping between input blocks of 160 speech samples in 13 bit uniform PCM format into encoded blocks of 112 bits and from encoded blocks of 112 bits to output blocks of 160 reconstructed speech samples. The sampling rate is 8 000 sample/s leading to an average bit rate for the encoded bit stream of 5,6 kbit/s. The coding scheme is called Vector Sum Excited Linear Prediction (VSELP) coding.

GSM 06.20 [4] describes the codec and GSM 06.06 [11] defines the C code, thus enabling the verification of compliance to GSM 06.20 [4] to a high degree of confidence by use of a set of digital test sequences given in GSM 06.07 [10].

6 Half rate Discontinuous Transmission (DTX)

The half rate discontinuous transmission function is described in GSM 06.41 [7].

During a normal conversation, the participants alternate so that, on the average, each direction of transmission is occupied about 50 % of the time. Discontinuous Transmission (DTX) is a mode of operation where the transmitters are switched on only for those frames which contain useful information. This may be done for the following two purposes:

1) In the MS, battery life will be prolonged or a smaller battery could be used for a given operational duration.

2) The average interference level over the air interface is reduced, leading to better Radio Frequency (RF) spectrum efficiency.

The overall DTX mechanism is implemented in the DTX handlers (Transmit (TX) and Receive (RX)) described in GSM 06.41 [7] and requires the following functions:

- a Voice Activity Detector (VAD) on the TX side, see GSM 06.42 [8];

- evaluation of the background acoustic noise on the TX side, in order to transmit characteristic parameters to the RX side, see GSM 06.22 [6];

- generation of comfort noise on the RX side during periods where the radio transmission is turned off, see GSM 06.22 [6].

ETSI GSM 06.02 version 6.0.1 Release 1997 10 EN 300 966 V6.0.1 (1999-06)

The transmission of comfort noise information to the RX side is achieved by means of a SIlence Descriptor (SID) frame. The SID frame is transmitted at the end of speech bursts and serves as an end of speech marker for the RX side. In order to update the comfort noise characteristics at the RX side, SID frames are transmitted at regular intervals also during speech pauses. This also serves the purpose of improving the measurement of the radio link quality by the Radio SubSystem (RSS).

The DTX handlers interwork with the RSS using flags. The RSS is controlled by the transmitter keying on the TX side, which performs pre-processing functions on the RX side. This is described in GSM 06.41 [7].

The speech flag (SP) indicates whether information bits are speech or SID information. The SP flag is calculated from the VAD flag by the TX DTX handler. When SID information is transmitted (SP="0") the operation of the speech encoder is modified to reduce the remaining computation for that frame. This is described in GSM 06.22 [6].

7 Half rate Voice Activity Detection (VAD)

The half rate VAD function is described in GSM 06.42 [8].

The input to the VAD is a set of parameters computed by the half-rate speech encoder defined in GSM 06.20 [4]. The VAD uses this information to decide whether each 20 ms speech coder frame contains speech or not.

NOTE: The VAD flag is an input to TX DTX handler and does not control the transmitter keying directly.

GSM 06.42 [8] describes the VAD algorithm and GSM 06.06 [11] defines the C code. The verification of compliance to GSM 06.42 [8] is achieved by use of digital test sequences (see GSM 06.07 [10]) applied to the same interface as the test sequences for the speech codec.

8 Half rate comfort noise insertion

The half rate noise comfort insertion function is described in GSM 06.22 [6].

When switching the transmission on and off during DTX operation, the effect would be a modulation of the background noise at the receiving end, if no precautions were taken. When transmission is on, the background noise is transmitted together with the speech to the receiving end. As the speech burst ends, the connection is off and the perceived noise would drop to a very low level. This step modulation of noise may be perceived as annoying and reduce the intelligibility of speech, if presented to a listener without modification.

This "noise contrast effect" is reduced in the GSM system by inserting an artificial noise, termed comfort noise, at the receiving end when speech is absent.

The comfort noise processes are as follows:

- the evaluation of the acoustic background noise in the transmitter;

- the noise parameter encoding (SID frames) and decoding;

- and the generation of comfort noise in the receiver.

The comfort noise processes and the algorithm for updating the noise parameters during speech pauses are defined in detail in GSM 06.22 [6].

The comfort noise mechanism is based on the half rate speech codec defined in GSM 06.20 [4].

9 Half rate lost speech frame substitution and muting

The half rate speech frame substitution and muting function is described in GSM 06.21 [5].

ETSI GSM 06.02 version 6.0.1 Release 1997 11 EN 300 966 V6.0.1 (1999-06)

In the receiver, frames may be lost due to transmission errors or frame stealing. GSM 06.21 [5] describes the actions to be taken in these cases, both for lost speech frames and for lost SID frames in DTX operation.

In order to mask the effect of an isolated lost frame, the lost speech frame is substituted by a predicted frame based on previous frames. Insertion of silence frames is not allowed. For several subsequent lost frames, a muting technique shall be used to indicate to the listener that transmission has been interrupted.

10 Half rate codec homing

The GSM half rate speech transcoder, VAD, DTX system and comfort noise parts of the audio processing functions (see figure 1) are defined in bit exact arithmetic. Consequently, they shall react on a given input sequence always with the corresponding bit exact output sequence, provided that the internal state variables are also always exact in the same state at the beginning of the experiment.

The input test sequences provided in GSM 06.07 [10] shall force the corresponding output test sequences, provided that the tested modules are in their home-state when starting.

The modules may be set into their home states by provoking the appropriate homing-functions.

NOTE: This is normally done during reset.

Special inband signalling frames (encoder-homing-frame and decoder-homing-frame described in GSM 06.20 [4] have been defined to provoke these homing-functions also in remotely placed modules.

This mechanism is specified to support three main areas:

- type approval of mobile terminal equipment;

- type approval of infrastructure equipment;

- remote control and testing for operation and maintenance.

At the end of the first received homing frame, the audio functions that are defined in a bit exact way shall go into their predefined home states. The output corresponding to the first homing frame is dependent on the codec state when the frame was received. Any consecutive homing frames shall produce corresponding homing frames at the output.

ETSI GSM 06.02 version 6.0.1 Release 1997 12 EN 300 966 V6.0.1 (1999-06)

Annex A (informative): Change Request History

Change history

SMG No. TDoc. CR. No. Section New Subject/Comments No. affected version SMG#13 4.0.2 ETSI Publication SMG#20 5.0.1 Release 1996 version SMG#27 6.0.0 Release 1997 version SMG#28 6.0.1 ETSI Publication

ETSI GSM 06.02 version 6.0.1 Release 1997 13 EN 300 966 V6.0.1 (1999-06)

History

Document history Edition 1 December 1996 Unified Approval Procedure UAP 60: 1996-12-02 to 1997-03-28

Edition 1 May 1997 Publication

V6.0.0 January 1999 One-step Approval Procedure OAP 9922: 1999-01-29 to 1999-05-28

V6.0.1 June 1999 Publication

ISBN 2-7437-2919-8 Dépôt légal : Juin 1999

ETSI